The Jefferson river is a tributary of the Missouri River, in the state of Montana. The Jefferson River and the Madison River form the official beginning of the Missouri at Missouri Headwaters State Park near Three Forks. It is joined 0.6 miles downstream by the Gallatin River.

The Jefferson river, the western most fork at Three Forks was named by Meriwether Lewis while he camped on the banks of the river during July 1805. The Jefferson River, the western fork and the largest of the three forks, was named for President Thomas Jefferson. The eastern fork, the Gallatin was named for the standing U.S. Treasury Secretary, Albert Gallatin, who served in such capacity from 1801-1814. The middle fork was named for James Madison, the Secretary of State.

From the high elevations of the Rocky Mountains of southwestern Montana, three small rivers converge to form the headwaters of the Jefferson River. The longest of these three waterways begins at Brower's Spring, 9,030 feet (2,750 m) above sea level, on the northern flank of the Centennial Mountains. The site is marked by a pile of rocks.[5] The water flows west then north as Hell Roaring Creek before merging with Rock Creek and flowing west through Upper and Lower Red Rock Lakes. Here it becomes the Red Rock River, flowing west through Lima Reservoir and then northwest into Clark Canyon Reservoir near Dillon. Below the dam, the river is known as the Beaverhead River. It is joined by the Ruby River above the town of Twin Bridges and converges with the Big Hole River to form the Jefferson about two miles downstream from town.[6]

Missouri at Three Forks, Montana, 45.926679, -111.505068

Junction with Big Hole River and Beaverhead River. 45.561972, -112.337682

63 miles

Montana: Big Hole River, Hells Canyn Creek, Cherry Creek, South Boulder River, Milligan Creek, Three Forks Junction.

Montana: Beaverhead River, Bone Basin Creek, Big Pipestone Creek, White Deer Creek, Conrow Creek, Boulder River, Antelope Creek, Sand Creek, Willow Creek, Three Forks Junction.

Upon leaving Toston, Montana, I travel south only anther twenty miles and reach the end of the Missouri river at Three Forks, Montana, or as it is known, the source of the Missouri. However, at the small state park where Lewis and Clark camped during 1805, there are three other rivers that feed the Missouri river. These three rivers are the Gallatin, the Madison and the Jefferson rivers.